The process can be applied to pulps from alkaline digestion, e.g. by kraft, counter-current kraft, polysulfide, soda and alkaline sulphite digestion or from digestion at low alkalinity and in acid media, e.g. processes in which sulphite is employed. Before the process according to the invention, the pulp can be screened and the liquor from the digestion removed by known techniques, such as pressing, filtration and washing, more or less completely depending on the starting material, the end-use of the pulp and the envisioned emission of organic solutes which all must be considered also in the choice of the bleach sequence. The end-use can vary from semibleached pulp for packages to almost lignin-free dissolving pulps produced with a large decrease in viscosity which is in this document applied as a measure of the depolymerization of the cellulose. The process is well suited for production of paper pulps of high viscosity, brightness, and brightness stability with less chlorine content and water pollution than comparable pulps produced by previously known processes, e.g. those with Cl.sub.2 and with a large addition of chlorine dioxide.
It is well known that metal compounds from the wood, water and employed chemicals can catalyse the depolymerization of the cellulose and hemicellulose. The amounts of harmful metal compounds introduced into the oxygen bleaching depend on the wood, water and other materials and on corrosion. Metal determinations in the raw materials are used to adjust the conditions, e.g. before wood from new suppliers are employed. Magnesium compounds are widely used to suppress the depolymerization of the cellulose. A theory confirmed experimentally by many researchers is that harmful transition metal ions such as iron, copper, cobalt and manganese are coprecipitated with magnesium hydroxide in the alkaline medium and in this way deprived of their catalytic activity.
Soaking of the unbleached pulp with acid to remove harmful metal ions such as manganese before the oxygen bleaching has been applied (Rerolle et al., Pulp and Paper Intern., July 1969, pp 29-31; Smith et al., French Pat. 1 601 408, filed 1970).
Similarly, complexing agents such as aminopolycarboxylic acids EDTA, DTPA and related aminophosphonic acids have been employed, Per G. Lundgren et al. Swedish patent No. 8902058-0. Industrial applications have been reported in publications by Basta et al., (Tappi J. April 1990, 155; Proceed. Appita 1991, Symp. Wood and Pulping Chem., Vol. 1, p. 237) and by Bryant and Edwards (Proceed. Intern. Non-Chlorine Bleaching Conf. March 1994, Xmelia Island, Florida). The cost for environmentally acceptable applications of these techniques can be very high, unless the release of the complexing agents is accepted. Serious problems by the presence of manganese during oxygen bleaching were reported in the patent by Smith et al., while D'Ans et al. (Angew. Chem. 63, 368(1951); 65, 368 (1953), Abbot et al. (J. Pulp Paper Sci. 17, J10 (1991), and Colodette et al. (J. Pulp Paper Sci. 15, J45 (1989) report on problems with manganese catalysis during peroxide bleaches.
Extractions with both acids and complexing agents of these types in separate stages can be used to prepare pulps with extremely low metal contents. No industrial application of this method before bleaching with oxygen or active-oxygen containing compounds seems to have been published. In addition to the high cost for the chemicals and the abatement of the water pollution the pulps are extremely sensitive towards trace amounts of iron compounds. Pulps extracted under conditions leading to very low contents of transition metal compounds such as those of manganese, cobalt, copper and iron are attacked in an irreproducible manner during bleaching in chlorine-free stages. Hence, small variations in the transition metal contents can give rise to serious variations in the degree of delignification (decrease in kappa number), the brightness and viscosity after treatments in stages with oxygen and other active-oxygen containing compounds. These variations are extremely serious for the quality of e.g. viscose pulps and paper pulps for fine paper of extremely high brightness, brightness stability, strength and viscosity and at the same time low carbonyl contents. The reported viscosities refer to intrinsic viscosity, dm.sup.3 /kg, according to SCAN 15:88. The decrease in viscosity during the oxygen bleaching is used as a measure of the depolymerization of the cellulose. The kappa numbers were determined according to SCAN 1:77.